The present invention relates to a charged particle beam irradiator and, more particularly, to a charged particle beam irradiator suitable for obtaining a scanned image of a sample at a high resolution and preferable contrast.
To obtain a scanned image of a sample at a high resolution especially with a scanning electronic microscope, it has been necessary to make primary electron beams scanned on the sample as narrow as possible. The most popular means for making primary electron beams scanned on a sample narrow is a method wherein the focal length of the objective lens is reduced to reduce the aberration of the lens.
One well known means for achieving a short focal point is an in-lens type electromagnetic lens in which a sample is located between magnetic poles of an objective lens (e.g., that disclosed in Japanese unexamined patent publication No. S62-291849).
However, limitations are placed on an in-lens type electromagnetic lens including a limitation on the moving range of a sample associated with the configuration in which the sample is interposed between the magnetic poles of the lens. For example, while some recent scanning electronic microscopes include a tilted stage in a sample chamber to allow the sample to be observed at an inclination depending on the object of the observation, the movement is limited by the presence of the magnetic poles located above and under the sample. Meanwhile, a sample must be located close to the upper magnetic pole in order to reduce the chromatic aberration coefficient and spherical aberration coefficient and, which is a factor that conversely reduces the area in which the sample is tilted.
Further, even an apparatus in which a sample is moved only in a direction perpendicular to electron beams, a problem arises in that it is limited with regard to the space to install it because of its configuration wherein a moving mechanism thereof is provided in an objective lens.
It is an object of the present invention to solve those problems and to provide a charged particle beam irradiator including an electromagnetic lens with which a small aberration coefficient can be achieved while reserving a sufficient tilting range for samples and a space for installing a sample moving mechanism and the like.
In order to achieve the above-described object, according to the present invention, there is provided a charged particle beam irradiator comprising an electromagnetic lens which includes a magnetic pole provided between a sample and a charged particle source and at least a pair of magnetic poles provided under the sample and which is configured to generate a magnetic field therebetween.
With the above-described configuration, it is possible to converge charged particle beams with a magnetic field generated between the magnetic pole provided between the sample and the charged particle source and the magnetic poles provided under the sample and to limit the spread of the magnetic field distribution of the magnetic field for converging said charged particle beams toward the charged particle source with a magnetic field generated between the at least two magnetic poles provided under the sample.
This can be more specifically described as follows.
Referring to FIG. 2, when a magnetic field is generated between an upper magnetic pole and another magnetic pole by a magnetizing coil A, the magnetic field distribution of the same will be as indicated by (A) in FIG. 3. Meanwhile, when a magnetic field is generated between magnetic poles under the sample by a magnetizing coil B, the magnetic field distribution of the same will be as indicated by (B). When the respective magnetizing currents are reversed with the strength of the magnetizing coils A and B set such that the tails of the magnetic field distributions (A) and (B) generally agree with each other in strength, the distributions (A) and (B) are synthesized into a distribution (C). Since the spread of the distribution tail (spread toward the electron source) is significantly suppressed in the magnetic field distribution (C), a very short focal length is achieved while reserving some distance between the upper magnetic pole and the sample (while reserving an operating distance for the sample or a sufficient space for providing a sample moving mechanism).
Although magnetic field strength required for focusing may not be obtained with the synthesized magnetic field distribution (C) when the acceleration voltage is increased, the required magnetization strength can be achieved in this case by setting the current ratio between the magnetizing coils A and B such that the strength of the magnetic field distribution (B) is slightly reduced. While this makes the focal distance slightly longer to increase chromatic aberration, the reduction in resolution due to the increase in chromatic aberration is negligible because chromatic aberration contributes less to resolution at a high acceleration voltage. While an increase in the focal length results in an increase in the spherical aberration coefficient, no practical problem occurs even if the spherical aberration coefficient is increased to some extent because resolution is reduced only at a factor of the quadruple root of the spherical aberration coefficient.
In addition, it is possible to detect a signal that provides desired contrast efficiently without decreasing the operating distance by providing a secondary electron acceleration electrode, a secondary electron conversion electrode (for detecting an electron signal backscattered at low acceleration), orthogonal electromagnetic field generating means (for detecting secondary electrons efficiently) and a backscattered electron detector (for detecting an electron signal backscattered at high acceleration) closer to the electron source than the objective lens in order to detect secondary electrons and backscattered electrons selectively and efficiently.
As described above, present invention makes it possible to reserve a distance required to move a sample and to provide a moving mechanism while achieving a short focal length and, in addition, to achieve display of an image at a high resolution and preferable contrast with an operating distance reserved to allow a sample to be tilted because a signal which contributes to the desired contrast can be detected selectively and efficiently.